1. Field of the Invention
The present invention relates to an optical signal processing device suitable for all-optical regenerative repeat.
2. Description of the Related Art
A Mach-Zehnder interferometer (MZI) type optical gate is known as a conventional waveform shaping device for performing waveform shaping at the optical level. This optical gate is configured by integrating a Mach-Zehnder interferometer including first and second nonlinear optical media each for providing a phase shift on an optical waveguide substrate, for example. Probe light as continuous wave (CW) light is equally divided into two components, which are in turn supplied to the first and second nonlinear optical media. The optical path length of the interferometer is set so that output light is not obtained by interference of the two components of the probe light.
An optical signal is further supplied to one of the first and second nonlinear optical media. By properly setting the powers of the optical signal and the probe light, a converted optical signal synchronous with the optical signal is output from the optical gate. The converted optical signal has the same wavelength as that of the probe light.
It has been proposed to use a semiconductor optical amplifier (SOA) as each of the first and second nonlinear optical media. For example, an InGaAs-SOA having opposite end faces treated with antireflection coatings is used as each nonlinear optical medium in a 1.5 xcexcm band, and these nonlinear optical media are integrated on an InP/GaInAsP substrate to fabricate an optical gate.
A nonlinear optical loop mirror (NOLM) is known as another conventional waveform shaping device. The NOLM includes a first optical coupler including first and second optical paths directionally coupled to each other, a loop optical path for connecting the first and second optical paths, and a second optical coupler including a third optical path directionally coupled to the loop optical path.
By forming a part or the whole of the loop optical path from a nonlinear optical medium and supplying probe light and an optical signal respectively to the first optical path and the third optical path, a converted optical signal is output from the second optical path.
An optical fiber is generally used as the nonlinear optical medium in the NOLM. In particular, a NOLM using a SOA as the nonlinear optical medium is referred to as a SLALOM (Semiconductor Laser Amplifier in a Loop Mirror).
In an optical fiber communication system that has been put to practical use in recent years, a reduction in signal power due to transmission line loss, coupling loss, etc. is compensated by using an optical amplifier such as an erbium doped fiber amplifier (EDFA). The optical amplifier is an analog amplifier, which functions to linearly amplify a signal. In this kind of optical amplifier, amplified spontaneous emission (ASE) noise generated in association with the amplification is added to cause a reduction in signal-to-noise ratio (S/N ratio), so that the number of repeaters (optical amplifiers) is limited to result in the limit of a transmission distance. Further, waveform degradation due to the chromatic dispersion owned by an optical fiber and the nonlinear optical effects in the fiber is another cause of the transmission limit. To break down such a limit, a regenerative repeater for digitally processing a signal is required, and it is desirable to realize such a regenerative repeater. In particular, an all-optical regenerative repeater capable of performing all kinds of signal processing at the optical level is important in realizing a transparent operation independent of the bit rate, pulse shape, etc. of a signal.
The functions required for the all-optical regenerative repeater are amplitude restoration or reamplification, waveform shaping or reshaping, and timing restoration or retiming. These functions are referred to as 3R functions, and in particular, the first and second functions are referred to as 2R functions.
The 2R functions can be provided by combining a waveform shaper and an optical amplifier, or by using a waveform shaper having an optical amplifying function. Further, the 3R functions can be provided by additionally using a clock regenerator in parallel to the 2R functions.
In the case of using an optical-level time division multiplexing (OTDM) type regenerative repeater for WDM (wavelength division multiplexed) signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths, there is a problem such that unless the modulation-phases of the plural optical signals to be input into the repeater have stable temporal order, stable regeneration processing cannot be performed.
It is therefore an object of the present invention to provide an optical signal processing device which can give stable temporal order to the modulation-phases of the plural optical signals of WDM signal light.
In accordance with an aspect of the present invention, there is provided an optical signal processing device comprising an optical demultiplexer having an input port and a plurality of output ports, said input port being adapted to accept WDM signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths; an optical multiplexer having an output port and a plurality of input ports; a plurality of optical paths for respectively connecting said plurality of output ports and said plurality of input ports; at least one delay adjuster provided on at least one of said plurality of optical paths; a detector for detecting the modulation-phase of at least one of said plurality of optical signals; and a controller for controlling said delay adjuster according to said modulation-phase detected by said detector.
With this configuration, the controller controls the delay adjuster according to the above-mentioned condition, thereby allowing stable temporal order to be given to the modulation-phases of the plural optical signals of the WDM signal light.
In accordance with another aspect of the present invention, there is provided an optical signal processing device comprising a phase adjusting section and an all-optical regenerating section, said phase adjusting section comprising an optical demultiplexer having an input port and a plurality of output ports, said input port being adapted to accept WDM signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths; an optical multiplexer having an output port and a plurality of input ports; a plurality of optical paths for respectively connecting said plurality of output ports and said plurality of input ports; at least one delay adjuster provided on at least one of said plurality of optical paths; a detector for detecting the modulation-phase of at least one of said plurality of optical signals; and a controller for controlling said delay adjuster according to said modulation-phase detected by said detector.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.